Combustion analysis carried out by a QinetiQ scientist, on behalf of the BOI team, has confirmed that high altitude fuel ignition by electricity is impossible without High Energy sparks. This leaves only pure heat as the cause of the ignition. The only heat source near to fuel pipes and tanks in a Nimrod are the engines (with fire extinguishers) and hot air pipes. All hot air pipes that are not in the immediate vicinity (and therefore protected by the fire extinguishers), have not been in airborne use since 2 Sep 06. This includes the SCP, the pipe that supplies it and the bomb bay heating system. It is considered that the bomb bay heating system pipes do not carry air hot enough to ignite any fuel playing onto them, so that system might be brought back into service.

There was a comprehensive post-accident fuel system fire hazard analysis carried out by QQ in 2007. It confirms that, with the limitations put in place within days of the accident, there is sufficient mitigation against a fire caused by a fuel system leak. To quote someone, " The Nimrod is as safe as it needs to be". He was correct.

It is the simple fact that the RAF was able to mitigate against the cause of a fuel fire within days of the accident by procedure alone (and was therefore capable of doing so before the accident, but didn't, for reasons yet to be established) that the S of S had no choice but to say sorry and offer compensation. We knew, with our radical and far-reaching limitations, that we had eliminated all possible causes, although we didn't know the probable point of ignition until the BOI reported its findings.

Ed

EdSet100,

Thank you for your explanation.

May I ask you if there is any possibiliy (albeit a remote one) that fuel leaking from inside the bomb bay (or at least partly 'originating' from there) could migrate out side of the fuselage and into the engines exhaust stream?

And if it could, is there any possibility that it could be ignited by the jet eflux as it vapourises into the atmosphere? (almost like the F-111 fuel dump system)

Thank you
The Winco

Winco,

As to your previous post, not a chance. Firstly the fuel would have to overcome the air moving over the airframe at x Hundred Knots and also overcome gravity as the Engines exhaust is above the bomb bay. Secondly On start up of the Spey 250 there is large amounts of fuel vapour present at the exhaust, And I have never witnessed it combusting.

Hope this helps.

Winco,
To add to enginesuck's answer, yes, fuel has been known to migrate along the underside of the wings or the fuselage, but only the fuel streaming on the wing skin underneath the jet pipes will flow off into the jet efflux. Fuel on the outside of the fuselage originating from the bomb bay would have to travel upwards and outwards to reach the underside of the jet pipes. This would have to occur within the very thin boundary layer of air under the relative airflow, which means that the amount of fuel in that layer would have to be very small and go against gravity to get up under the jet pipes. Any large leak would be caught in the RAF and disappear.

I have heard of fuel lightly spraying into the jet efflux due to fuel leaks. None of those incidents resulted in a fire in the jet efflux.

The F-111 uses its reheat system to start the fire, which naturally projects rearwards into the RAF. When the pilot selects DUMP, he simply adds fuel to an existing fire.

Hope this helps

Ed Sett

Thank you Ed,

I read somewhere in the accident report about the fuel migrating to outside of the skin and along the fuselage, and whilst I can see that the direction of the fuel travel would be against gravity and airflow, I was more interested in the remote possibility of fuel getting into the jet eflux during AAR when, the aircrafts attitude is 'nose up' (if I remember correctly) and the various wake turbulances and vortexs generated from the tanker can cause unusual airflows and currents?

Thanks for the explanation.

The Winco

Yes, that was an explanation of one of the probable causes of fuel getting onto the very hot SCP pipe. Fuel could have exited the No1 Tank blow-off valve, which is on the stbd fuselage just under the wing joint, and run down the side of the fuselage and then into the area of the SCP pipe. Combustion analysis has proved that the small amount of fuel (300ml) that could have been retained in that area up against the SCP pipe, would have been sufficient to start a fire big enough to damage the No7 tank fuel pipes (above the SCP pipe) that would then add more fuel to the fire.

Ed

EdSet100: Yes you are 100% correct about the SCP being disconnected at the time of the XV260 incident in Nov 2006. I had overlooked that fact. However, I am not sure that you are correct to say With all the restrictions and pre-flight test in place, we still have an incident like the one on 5th Nov 2007.

I do share your concerns that action to isolate the SCP immediately after the crash was too late for XV230. The question to be asked, is why was the system reactivated after the XV227 incident?

Perhaps you can suggest how fuel from No.1 blow-off, could migrate upwards to No.7 tank dry bay.

Finally, do you have a date for the QQ hazard report, as it does not appear to be mentioned in the BOI report. I note that in the BAE hazard report, power supplies are mentioned as possible ignition sourses.

Many thanks for the information supplied so far.

DV

DV, look at more of EdSett's comment, I would guarantee that no one will sign to say that the aicraft is completely safe !

This may seem like splitting hairs, but people have to understand the reality of the situation, and the reality is that nothing is completely safe. The objective should be set in terms of an acceptable level of safety, and there are various ways of doing this if you look at different industries.

Regards

S_H

DV,

The 227 incident was caused by a ruptured SCP pipe. All of those pipes on the other frames were replaced following that incident, thus eliminating the possibility of it happening again. So, the SCP was re-introduced as it was believed safe to operate again. The question that should be asked is why was the hazard of the 1 tank blow off being in proximity to the pipe not noted earlier?

Hindsight is a wonderful thing, this is an aicraft that was built decades ago, in a different safety regime. The designers missed the hazard, A&AEE missed the hazard, the Ground Engineers missed the hazard, us aircrew missed the hazard. Indeed, 25 years of AAR never highlighted the hazard until 230. So, let's not be too hasty pointing the finger of guilt at any one person.

Sorry Vage Rot the section of piping replaced after the XV227 incident was a very small section of a network of suspect piping, associated with the SCP. That is why a survey of other pipes was requested. Someone jumped before the true state of SCP system was known. I am prepared to point my finger at who ever made that call.

DV

I accept that I shouldn't have said "completely" safe. I meant to indicate that the Nimrod should not (now) be viewed, in safety terms, in a different light to any other aircraft.

The incident on 5 Nov is subject to an ongoing investigation and I will not comment.

The fault with the SCP duct on 227 was a duct failure. That particular fault was addressed and recommendations were made to examine other similar ducts, for the same symptoms. No-one considered that a perfect set of SCP pipes, and the pipes supplying the SCP, that carry air at 400C would get covered with fuel. That was a different scenario to the burst duct. So, the SCP was cleared for unrestricted use once it had been repaired.

Diagrams that you might have seen tend to indicate that the 7 Tank dry bay doesn't extend to the bottom of the wing root. The bay floor is actually the underwing panel that shrouds the SCP pipe. Therefore, the fuel didn't need to migrate upwards. It simply flowed rearwards and passed through the small gap between the front edge of the panel and the airframe.

I understand that QQ was commissioned by the IPT to carry out the study in Apr 07. It was not commissioned by the BOI. I believe the BOI had already seen the flawed Nimrod Safety Case and, as required by their Terms of Reference, reported their findings to the IPT. The IPT then commissioned QQ to conduct the hazard analysis. The report was eventually submitted to the IPT in Sep 07.

I haven't seen that report, so I can't really comment. However the QQ combustion analysis would address those "possible" sources. Taking your words verbatim, BAe might have suggested that power supplies could possibly cause electrical fires, or possibly cause fuel fires. Well, yes, it was possible until proven otherwise. I believe that subsequently the QQ combustion report proved otherwise in respect of the fuel fire scenario.

I think thats a little bit unfair. Even if all of the similar pipes had been replaced with the SCP pipes, even as a precaution, the fact is that all of those pipes and the SCP would have been operating on 230 on that day. Indeed, it can be argued that changing the pipes would create possibilities of introducing leakage at the disturbed joints. So, IMHO the condition of the pipes was not contributory. However, it is disappointing that the recommendation to form a maintenance policy for the SCP and other similar ducts has taken so long to be decided on. Some of those similar ducts run very close to fuel tanks. Fortunately, none of those similar ducts are currently pressurised in flight.

Ed

EdSet100: You state
Does this new hazard analysis negate the complete BAE systems hazard analysis of 2004, and which IPT approved in Feb 05? Or does it only address the problems in the dry bay area?

I think we have got to remember that the BOI only "suggested" that the fuel source was the blow-off, it could equally have been caused by a leaking coupling (and we have had many of those). They also suggest the probem could have been caused by a small hole in a hot air pipe, which in turn damaged a fuel line. (very much like XV227). It is also worth reading Air Member for Materiel's comments in the BoI report.
Whilst we still have problems with fuel in the bomb bay post AAR, I can not see how we can consider the aircraft safe.

Once again, many thanks for your very informative comments

DV

No problem. I don't think I'm breaking any rules in discussing and amplifying info already in the public domain. Although some of it should not have been released.

Good question, and I'm sure its one that the QC will be posing in due course. Its the most accurate analysis we have at the moment and if it differs from any previous analyzes on points of fact, it has to override them in those particular matters. It covered all aspects of the Nimrod fuel system.

The SCP and the other pipes in that zone are surrounded by heat sensors, specifically designed and located to detect hot air leaks. So, the BOI rightly surmised that any hot air leak would have been below the detection threshold (235C), because the engineer did not mention an overheat warning. Such a small leak would have to have played directly onto a fuel coupling (it would never have burned through a metal pipe) such that the coupling failed and the fuel then played onto the pipe. Its a series of events that are less likely than a simple coupling leak (for any reason) or a blow-off. However, its a scenario that can't be discounted. I should add that a number of the "similar" pipes, as referred to in the 227 incident, on other aircraft have since been checked for deterioration and none has been found showing any potential to fail.

Well, the BOI used the term, "probable cause". That leaves the door open for anyone to add the quoted comment. However, I place my personal trust, and I recommend it to other aircrew, in the QQ combustion analysis sponsored by the BOI. It provides us with the confidence that fuel leaks in the bomb bay are not dangerous in the absence of very hot surfaces. I'm not suggesting that the crews should ignore fuel leaks, but we need to be confident in that particular analysis, respect our ground crew's efforts to prevent the leaks and rely on our training to deal with them if they occur. We have well-practised procedures to stop every single fuel pipe leak on a Nimrod.

Ed

Air-pipe failures add to spy plane safety fears
 

From The Sunday Times
January 13, 2008
Air-pipe failures add to spy plane safety fears

Michael Smith
CONCERNS over the safety of the RAF’s Nimrod spy planes have increased after it emerged that hot-air pipes in the aircraft’s engines could fail at any time.

The pipes failed during “low pressure tests”, according to a report by the plane’s manufacturer, BAE Systems.

The company had warned previously that the pipes were close to fuel lines and that if they failed in the air they would be likely to start a fire, leading to a “catastrophic” incident.

The new safety fears come a month after Des Browne, the defence secretary, apologised to the families of 14 servicemen killed when their Nimrod, XV230, caught fire over Afghanistan.

BAE’s report on the tests says 14 hot-air pipes it was sent to check already had cracks in them and eight were seriously corroded.

The report, obtained under the Freedom of Information Act, does not appear to have been seen by the board of inquiry into the loss of Nimrod XV230, which published its damning report last month.

One of the hot-air pipes that was already cracked was part of the system on XV230. It was a junction pipe leading to the hot-air pipe that set fire to leaking fuel. This caused the explosion that destroyed the plane.

Jimmy Jones, a former RAF Nimrod engineering officer, said it was “extremely unlikely” the board would not have mentioned a crack in this area if it had known about it.

The board’s report also refers to the BAE pipe tests as if they are continuing, when in fact BAE’s findings were published last February, months before the inquiry came to an end.

Leaks in pipes from engines are worrying as most fuel leaks on the Nimrod are from the wing tanks around the engines.

“It is highly questionable whether Nimrod should be flying with potential hot-air leaks in the wing tanks,” Jones said.

The Ministry of Defence said it did not want to preempt an inquiry into the safety of the Nimrod fleet, but some hot-air piping had been isolated.

Now normally Michael Smiths article seem fairly technically accurate, but this is really really wide of the mark as far as i can see. Hot air pipes in the engines???
Maybe he is refering to air offtake pipes ? As for air offtake pipes being close to fuel pipes, name me one modern gas turbine engine with which this isn't the case. And as for Jimmy bloody Jones, where exactly are these hot air leaks in the wing tanks? Buffoon.

enginesuck

The article should have said pipes in the engine bay areas. Similarly, the Jimmy Jones quote has for some reason been completely distorted during the production process by the idiotic removal of the word “areas”. :ugh:

What he actually said was: “It is highly questionable as to whether Nimrod should now be flying with potentially hot air leaks in the wing tank areas." My apologies to him for that.

The part numbers for the pipes that failed were:
6M4E131A/1, described in the report as: “HP Air, Inner Engine LH” and
6M4E147A (7800), described as “Anti-icing duct, inner engine”.
The proof pressure given in the report was 575psi. Both were tested using water and failed at 100psi, which the report describes thus:

“Duct 10 (Part No 6M4E131A/1) and Duct 25 (Part No 6M4E147A) both leaked water through the bellows area at low pressure (<100psi).”

There is more detail here:
http://timesonline.typepad.com/mick_...fety-fear.html

enginesuck: I think you are missing the point. Here was a study conducted by BAE at the request of IPT, following the XV227 SCP duct incident, that never got to the BOI.

DV

Thank you Mick Smith & DV, I will raise the question of this report with the Coroner at the Pre-Inquest tomorrow

It is a fact of aviation life that engines need fuel and those same engines produce very hot air for other systems. So, it is inevitable that we will have a cocktail of ingredients for a fire, on most aircraft types worldwide, in engine compartments. So, we install fire warning and extinguishant systems. We have hot gas leak detectors in those compartments and we have aircrew trained to deal with these situations. The Nimrod includes all of these measures.

The pipes in question are either subject to the above safety measures or they are not now in use while airborne.

The cracked junction by the SCP, specifically mentioned in the BAe report, is monitored by a hot gas leak detector. There are 5 detectors in that area. The BOI were fully aware of the possibility of a duct failure in the area of the fire and didn't need to see the report anyway. It would only have confirmed their findings.

Mick, I admire your tenacity in this sublect, but we must be aware that tests and studies of this nature are very narrow in their scope and they set out to address only the questions that are posed by the sponsor. The wider issue of what happens to the aircraft when a pipe leaks is not within the scope of that BAe report and it cannot be used as the sole arbiter of aircraft safety. It is a quantum leap in logical thought to suggest that an air pipe failure (which I guess the report suggests is not unlikely) in a Nimrod engine bay will cause a catastrophe and therefore makes the aircraft unsafe. Of course, the ducts will leak/fail. We will know about it immediately and we are well prepared to deal it.

Ed Sett

EdSet100; Once again many thanks for your very open and honest comments. They generate heathy depate.

You say
Can I remind you of what the BoI said about this junction in their report.
There is no mention of "The Board is aware of the a possible duct failure in this area". Also, if as you say, the Board was aware the possibility, then it has to be assumed that IPT were aware of it when the system was reactivated. If that is the case, then they played with people's lives. You may have 5 detectors, but by the time you could have done anything you had an uncontrolled fire.

I also suggest that you read the BAe Harzard report, which highlights the threat in the engine bay areas from hot air.

DV